Research On Nano Friction Characteristics Of Graphene Surface With Stick-slip Based On AFM

Graphene,as an ultrathin two-dimensional material of nanometer scale,has extremely high in-plane strength and excellent lubricating performance.When the thickness of graphene drops to an atomic layer,the graphene surface still has outstanding low friction ability and is the first choice of ultrathin lubricating film in micro/nano electromechanical systems(M/NEMS).M/NEMS has different requirements for lubricating materials under different application conditions such as frequent start-up and continuous operation,so it is necessary to fully study the friction properties of graphene surface.However,there are still many gaps in the study of surface friction of graphene,such as stick-slip motion,transient frictional strengthening,etc.Based on the above reasons,the friction characteristics of graphene surface were studied by atomic force microscopy(AFM):Firstly,the periodic frictional strengthening of graphene surface is mainly caused by the coupling of contact area evolution and contact mass evolution between the tipgraphene surface,and the wrinkle change and contact mass change during the stick-slip period are analyzed.Based on the FKT model,a physical model of tip sliding on suspended graphene is established.The periodic action potential is established at the tip atom,and the graphene atom is connected by a spring.The dynamic equation of frictional strengthening of the system is established.The errors of Ogletree method and Varenberg method in the theory of lateral force calibration in introducing adhesion force are pointed out,and the Ogletree wedge method is determined to be the optimal lateral force calibration theory for nano-friction experiments.Monolayer and multilayer graphene were prepared by mechanical exfoliation and characterized precisely by optical microscopy,Raman spectroscopy and AFM detection.Nano-scale friction experiments on graphene surface were carried out using transverse force microscopy(LFM).With the increase of velocity,the average friction force gradually decreases to a stable state,and when it reaches 3 layers,the speed dependence basically disappears.The viscous-slip friction(frictional vibration)will gradually decrease with the increase of the number of layers and reach stability in the 4 layers,and there is no load-dependent relationship.As the speed increases,the viscous-slip friction decreases in a small range,then increases rapidly,and then stabilizes.As the probe speed increases,the frictional strengthening strength will gradually decrease to a stable state and the saturation distance will gradually increase to a stable state.Frictional strengthening strength is independent of load,but the saturation distance increases with load.As the number of graphene layers increases,the frictional strengthening strength decreases rapidly to nearly 0,while the saturation distance is independent of the number of graphene layers.The torsional stiffness of the probe has a significant effect on the friction system of the substrate-graphene-probe.As the torsional stiffness increases,the average friction decreases in a small range and the velocity dependence decreases.At the same time,the viscous-slip friction will increase significantly,and change with the speed faster,can reach a stable at a smaller speed.With the increase of torsional stiffness,the strength of frictional strengthening obviously increases and the speed dependence becomes stronger.At the same time,the saturation distance will become smaller,and the speed dependence of the saturation distance will be weakened.As the load increases,the saturation distance of the probe with high stiffness increases less rapidly than that of the probe with low stiffness.Electrostatic force microscopy(EFM)was used to detect the electric field changes before and after the friction of graphene islands,and to determine whether EFM can be used as an effective detection method for the friction of graphene surface.In this paper,the surface friction characteristics of graphene have been systematically studied by using AFM,which provides theoretical support for the application of graphene as an ultrathin lubricating film.